Tailoring Magnetic Exchange Interactions in Ferromagnet-Intercalated MnBi2Te4 Superlattices

The intrinsic magnetic topological insulator MnBi2Te4 (MBT) has provided a platform for the successful realization of exotic quantum phenomena. To broaden the horizons of MBT-based material systems, we intercalate ferromagnetic MnTe layers to construct the [(MBT)(MnTe)m]N superlattices by molecular beam epitaxy. The effective incorporation of ferromagnetic spacers mediates the anti-ferromagnetic interlayer coupling among the MBT layers through the exchange spring effect at the MBT/MnTe hetero-interfaces. Moreover, the precise control of the MnTe thickness enables the modulation of relative strengths among the constituent magnetic orders, leading to tunable magnetoelectric responses, while the superlattice periodicity serves as an additional tuning parameter to tailor the spin configurations of the synthesized multi-layers. Our results demonstrate the advantages of superlattice engineering for optimizing the magnetic interactions in MBT-family systems, and the ferromagnet-intercalated strategy opens up new avenues in magnetic topological insulator structural design and spintronic applications.

In magnetic topological insulators (MTIs), the coexistence of broken time-reversal symmetry (TRS) and robust topologically protected surface states gives rise to a variety of emergent TRS-breaking physics [1][2][3][4][5] . One successful method to obtain MTIs is the incorporation of transition-metal elements (Cr, V, Mn) into the host TI matrix to establish the robust ferromagnetism 2,6,7 . However, the random distribution of magnetic atoms inevitably leads to a spatial fluctuation of the magnetic exchange gap, which restricts MTI-related phenomena at deep cryogenic temperatures. One approach to address such challenge is the design of TI-based magnetic heterostructures in which uniform magnetic order can be induced by an interfacial proximity effect 8 . In this way, the separation of the topology and the magnetism into different layers allows us to control their contributions independently. However, due to critical lattice-matching requirements, the selection of TI- 3 compatible magnetic films is limited and only a few MTI heterostructures have been fabricated [9][10][11] .
Alternatively, MnBi2Te4 (MBT), an intrinsic MTI material, has aroused extensive attention in recent years [12][13][14] . In contrast to magnetic doping, the Mn atoms in MBT strictly form a two-dimensional lattice plane within the stacked Te-Bi-Te-Mn-Te-Bi-Te septuple layers (SL). This structure not only preserves the large spin-orbit coupling required for band inversion, but also provides a homogeneous magnetic ground-state with intra-layer ferromagnetism (FM) and inter-layer A-type anti-ferromagnetism (AFM) 15 . As a result, dissipationless chiral edge conduction has been observed in thin MBT flakes above 1 K, although it requires the presence of a large external magnetic field to fully magnetize the samples 12,16,17 .
In this regard, new breakthroughs may be found through deliberate modifications of the MBT framework, from which spontaneous magnetization can be achieved at zero magnetic field 12 . For instance, recent attempts by inserting the Bi2Te3 spacer in (MBT)(Bi2Te3)n (n = 1-6) series crystals have been made, where the interlayer AFM-type Anderson super-exchange coupling strength is reduced significantly with increasing n [18][19][20][21][22] .
Furthermore, instead of this non-magnetic intercalation strategy, which may cause unintended magnetic fluctuations 23 , Mn2Bi2Te5 has been proposed as a superior candidate given that the additional Mn layer in the Mn-Te double spacer improves the magnetic stability of the original MBT SL. Unfortunately, during the Mn2Bi2Te5 single crystal growth, the Mn atoms tend to dope into MBT rather than forming the stable Mn-Te-Mn structure 15 , hence the experimental realization of FM-intercalated MBT proposals remains elusive 24 .
In this work, we utilize the molecular beam epitaxy (MBE) technique to integrate ferromagnetic MnTe and Experimentally, we first grew single-crystalline MBT films by depositing individual MnTe and Bi2Te3 layers on Al2O3(0001) substrates in an alternating sequence, followed by a moderate post-annealing procedure to form the MBT SL structure. The absence of dangling bonds on the Al2O3 surface promotes the epitaxial growth of MBT, and the sharp streaky pattern of the in-situ reflection high-energy electron diffraction (RHEED) reveals the two-dimensional growth mode of the as-grown MBT films (Fig. 1a). The cross-sectional high-resolution scanning transmission electron microscopy (HR-STEM) image of the MBT sample ( Fig. 1b) clearly unveils the highly ordered atomic distribution of the desired Te-Bi-Te-Mn-Te-Bi-Te sequence. In addition, the quantitative analysis of the electron energy loss spectrum (EELS) in Fig. 1c confirms an elemental composition ratio of Mn:Bi:Te = 1:2:4 in the single-crystalline film, and the X-ray diffraction (XRD) pattern ( Supplementary Fig. 1) exhibits a series of (00n) peaks without observable secondary phases, both of which agree well with the ideal stoichiometric MBT values. Through the polarized neutron reflectometry (PNR) measurements of the Te-capped MBT film, the nuclear scattering length density (SLD) profile is consistent with bulk-like MBT and the magnetic SLD diagram shows a uniform magnetization distribution across the film thickness, with a magnitude in accordance with that expected from MBT in an in-plane magnetic field, supporting high-quality growth of the designed phase ( Supplementary Fig. 2-1).
Subsequently, we performed four-point magneto-transport experiments on the 5 SL MBT sample to explore its magnetic/electrical properties. Consistent with studies of exfoliated MBT flakes 12,16,25 , the magnetoresistance displays a hump-like line-shape at T = 1.5 K, as highlighted in Fig. 1d. In particular, the initial antiparallel alignment of spins in adjacent MBT layers leads to the high-MR state in the low-field region, and the 5 abrupt increase in magnitude to a MR peak near a field of  3.3 T (i.e., defined as the transition field Ht) corresponds to the characteristic spin-flop feature for an A-type AFM system 12,16,25 . Upon further increasing the applied magnetic field, all magnetic moments become polarized along the z-direction at the saturation field of Hs = 7 T, and the MBT sample falls into a low-MR state. This behavior is similar to the giant magnetoresistance (GMR) effect found in a spin-valve system (the detailed GMR model is discussed in Supplementary Fig. 3). Besides, the corresponding anomalous Hall resistance (Rxy) data ( Fig. 1e) reaches saturation above Hs with Rs = 4 kΩ, a relatively larger value compared with other reported MBE-grown MBT films 15,26,27 . It is noted that the hybrid-AHE-like Rxy slope at low magnetic fields (± 3.3 T) may be caused by native anti-site defects and/or random stacking order in MBT, which in turn modify the local electronic structures and magnetic moments 22,24,26 . From the aforementioned structural and electrical characterizations, we have validated the high quality of our MBT layer as a reliable building block for the following study.
Based on our previous experience with the FM-type MnTe film growth 28 , we have chosen it as the intercalated layer to create the [(MBT)(MnTe)m]N system. As illustrated in Fig. 1f, the similar 2D stoichiometric structures with a small lattice mismatch between MnTe and MBT guarantees the epitaxial growth of the superlattice structure in which the strong ferromagnetism (i.e., Curie temperature of TC ~ 150 K) of MnTe can add robust FM interactions to the hybrid system, and the MnTe layer number m (i.e., the spacing between adjacent MBT layers) can be, in principle, utilized to tune the inter-layer AFM coupling. It is noteworthy that the maximum thickness of the MnTe layer in this study is limited below 1 nm (m = 1.75), which is sufficiently thin to ensure effective couplings among MBT layers in the superlattice 23,29 .
Given the known tendency of Mn to diffuse extensively in MBT-related systems, we further performed neutron reflectometry experiments to probe the superlattice depth and magnetic profiles.  Fig. 2-2a). Next, an additional PNR measurement was conducted at T = 6 K under the applied in-plane magnetic field of 3 T. of the AFM-related GMR-like behavior. By further increasing m, the FM contribution of the MnTe layer becomes more pronounced, therefore triggering the appearance of a double-split butterfly MR response from its original curve. Eventually, the overall MR profiles of the MnTe-dominated superlattice system (m ≥ 0.75) resembles that of the pure MnTe control sample except for a larger transition field (Ht). In fact, the mdependent Ht identified in Fig. 3a is reminiscent of the exchange-spring magnet behavior observed in conventional magnetic multilayers 30 . Depending on the relative strengths of the anisotropy and exchange coupling energy between constituent magnetic components, the ground-state spin-texture of the system and its correlated magneto-transport responses can be tuned 31  In addition to enabling the coercivity modulation, the establishment of exchange-spring effect is also found to mediate the interaction between adjacent MBT layers in our [(MBT)(MnTe)m]N superlattices. Unlike the mdependent Ht phenomenon noted in the low-field region, the saturation field Hs, which characterizes the interlayer exchange coupling strength, remains constant at 7 T (i.e., the same value as the non-intercalated MBT film) and shows a negligible variation with respect to m (i.e., the dashed straight line in Fig. 3a). In order to understand this distinctive feature, we carried out high-field anomalous Hall resistance measurements on the same set of [(MBT)(MnTe)m]5 samples at low temperatures. Strikingly, the normalized Hall resistance (Rxy/Rs) curves at T = 1.5 K (Fig. 4a) shows almost identical contours, indicating a universal transition process as the magnetic moments in the superlattice are gradually aligned by the applied field. In agreement with the MR results in Fig. 3, the saturation field Hs values extracted from the AHE data at T =1.5 K, 3 K, 5 K, and 10 K also stay independent on the MnTe spacer thickness (Fig. 4b). Similarly, the Hs-T slopes also follow the same temperature-scaling trace up to 25 K (Fig. 4c), above which the inter-layer AFM order disappears (i.e., Hs = 0) and the overall AHE signal reverts to that of a single-phase FM-driven Rxy hysteresis loop.
Quantitatively, the distinct evolutions of Ht and Hs versus the MnTe thickness can be well described by a modified linear chain model [36][37][38] , in which the magnetization of each constituent MBT and MnTe layer is considered as a 'macro spin'. As schematized in Fig. 5a, the magnetic interactions in the [(MBT)(MnTe)m]N superlattice system consist of the exchange couplings between nearest-neighbored MBT-to-MBT, MnTe-to-MnTe, and MBT-to-MnTe layers, respectively. Accordingly, the total energy EN of the system can be expressed as a function of the applied magnetic field H [36][37][38] 36 . Therefore, Equation (1) can be further expressed as the simplified formula: where ( +1 ) is the angle deviation between ( +1 ) and the magnetic easy-axis (i.e., z-direction), β and ξ are introduced as the scaling factors of the biquadratic coupling and different anisotropy, and is the MBT layer thickness (see Supplementary Information Section 4 for detailed discussion). From this modified linear chain model, it is found that the transition field Ht is affected by the value whereas the saturation field Hs is mainly decided by the magnitude (Supplementary Fig. 4). Consequently, to recapture the m-dependent MR behaviors observed in Fig. 3 Fig. 5b shows that the overall MR profile always maintains the GMR-like line-shape with slightly enlarged Ht and Hs fields as the MBT thickness varies from the 2D (4 SL) to quasi-3D (10 SL) region (the sample thicknesses are calibrated by Xray reflectivity (XRR) in Supplementary Fig. 6 and Table 6). The reduced GMR amplitude is possibly caused by the increased bulk conduction in thicker films 12,16 . On the contrary, with the insertion of the MnTe spacer, the magnetic proximity effect can re-orient the magnetic moments at the MBT/MnTe interface 34,35,42 . Under such circumstance, it is expected that a change in the number of superlattice repeats (i.e., the number of heterointerfaces) will introduce dimension-dependent features into the related magneto-transport results.  43 . As the repeat number increases, the added at the hetero-interfaces would promote the exchange-spring effect, as emphasized by the enlarged Ht field in Fig.   5c. Given that the adjacent MBT coupling is mediated through the intervening MnTe, the increase of superlattice repeats will cement such long-range interactions and further stabilize the new ground state. Along with the reinforced canted magnetization orientation which modifies the interfacial spin scattering, it is therefore unsurprising that a more prominent AFM-type GMR behavior is obtained in the N = 10 sample 8,44 . 11 In conclusion, we have demonstrated that combining ferromagnets with MBT facilitates the construction of intrinsic MTIs with configurable electronic structures and magnetic properties. In the FM-intercalated MBT systems, the versatile interfacial and interlayer magnetic exchange interactions offer an effective method to shape the overall spin texture, and the AFM-to-FM transition can be well-controlled by structural engineering.
Unlike the (MBT)(Bi2Te3)n compounds, the insertion of the MnTe spacer triggers the exchange spring effect which plays an indispensable role in the preservation of the global AFM coupling between adjacent MBT layers throughout the superlattice structures, and this advantage endows the system to be robust against external perturbation, which is favorable for device stability considerations. More importantly, the MR responses can also be tailored via superlattice periodicity optimization, thus providing another degree of freedom in the design of practical spintronic memory/sensor prototypes over traditional magnetic multilayers.
With the further exploration of exotic topological features embedded in the host MBT matrix, the